A favorite question of rabbis is, “Who is a Jew?” The answers of course vary with the rabbi, but all are focused on morality and conformance with Biblical law. Jon Entine, however, in his latest book, “Abraham’s Children,” focuses on recent studies of human DNA. When the Hebrews united under Moses about 4,000 years ago, they made his brother and aide-de-camp, Aaron, their chief priest. The office became hereditary, and Aaron’s descendants evolved into a priestly caste, Cohanim. Today’s telephone books are filled with Cohens, Cohns, Kohns, all according to tradition descended from Aaron.

In the early l990s a small group of Jewish geneticists, including Neil Bratman, a retired businessman; his son Robert, at that time a graduate student; and Karl Skorecki, a religious scientist; thought that a study of the genetic markings of today’s Cohens might be revealing, and that the field work necessary for such a study was well within their capabilities.

Joined by Michael Hammer, David Goldstein and Mark Thomas, they established their own laboratory and began a series of field studies that soon proved remarkably fruitful. One involved 300 Jewish males, one-third of whom identified themselves as Cohanim. In testing, the non-Cohanim were found to have a variety of genetic markers on their male chromosomes, none of which stood out.

In contrast, more than half of those claiming to be Cohanim shared a cluster of six identical markers that the researchers named the Cohen Modal Haplotype or CMH. What is notable is that these Cohanim included males from every corner of the Jewish Diaspora, whether Bombay, Kurdistan or Eastern Europe, and that although physically they might look much different from each other — skin color, eye color, hair structure — they all carried within their DNA identical genetic markings.

Other studies continued, which, when taking into account mutations, increased the percentage of those sharing the CMH to almost 98 percent. Naturally these findings provoked a great deal of publicity and some rather wild speculation. The researchers, and the author, have staunchly maintained that although their work does show a common male ancestor of all members of the Cohen family dating back some thousands of years it cannot prove the existence of Aaron, or Moses, or the story of the Exodus. Nevertheless it does strengthen credence in the Biblical account and gives us a glimpse of the possibilities of future DNA research.

The question of who is a Jew has been around for a long time and books have been written about the lost 10 tribes and their descendants. The Falashas, or black Jews of Ethiopia, are perhaps the most publicized. Completely isolated in the Ethiopian countryside, they nevertheless practiced Judaism, observed its holidays and conformed to its laws and rituals. They considered themselves Jewish and called themselves such.

Yet, DNA studies showed no link to ancient Israel. Whether this was important in determining their Jewishness, the fact is that a large number of them have settled in Israel with Rabbinic approval. Another tribe, the Lemba, found in Southern Africa, also considered themselves Jewish. Here DNA research ruled in favor of their beliefs since most of them were found to have a common male Semitic ancestor. This does not of course mean they were descended from the Israelites but does show an ancient connection to the Levant that could have involved Israel.

In reading this book one learns a good deal about the human genome and the efforts and goals of the researchers studying it. But, in his attempt to give us a full picture of the events comprising Jewish history, the author sometimes wanders. Thus we get a 17-page history of Hebron, which is relevant to the history of modern Israel but has little to do with genetic research.

Even so, it is good to read of the successes that seem to be becoming more frequent. We now are able to trace the onset of particular diseases to mutant genes that then make treatment more efficient. We may become even more healthy than we are now, and live to an even greater age — if that is desirable — through the research efforts this book describes.

The book ends with some fascinating appendixes. One lists “Jewish” diseases, and another shows how easy it is to determine one’s own DNA (you don’t have to be Jewish) — and includes a list of laboratories that make it possible and not exorbitantly expensive. All worth reading unless you are sublimely happy with the way you are now.